North Korea’s first-ever launch of an intercontinental ballistic missile poses a new threat to the United States, just as one of the Pentagon’s main weapons designed to defeat ICBMs gets back on track after a recent history of failures.

Pyongyang’s use of the two-stage Hwasong-14 ICBM on Tuesday appeared to catch the United States by surprise, and was initially labeled Monday night by the Pentagon as the testing of a intermediate-range missile. Navy Capt. Jeff Davis, a Pentagon spokesman, said Wednesday that the Defense Department reassessed its characterization Tuesday and identified it as an ICBM capable of ranging at least 5,500 kilometers, to Alaska. By then, independent experts had already determined it was an ICBM.

Davis said Wednesday that the missile “is not one we have seen before,” and that it was launched from a site — the Panghyon airfield about 90 miles north of Pyongyang — that has not been used to test missiles before. But he suggested that North Korea still has steps to meet before there is an imminent threat to North America, noting that Pyongyang has not yet demonstrated the ability to mount a nuclear warhead on an ICBM or the lateral range necessary.

“But clearly, they are working on it,” Davis said. “Clearly, they seek to do it. This is an aggressive research-and-development program on their part, and it is one that we have sought from the beginning with our ballistic missile defense system to be able to outpace it.”

The United States has built a web of missile defense weapons around the Pacific over the last few years, while continuing to call for a diplomatic solution. But one of the main cogs in it — missile interceptors known as the ground-based midcourse system — have struggled in testing. The Pentagon’s top weapons tester, the Office of the Director of Operational Test and Evaluation, assessed last year that it had demonstrated only a limited ability to defend the United States, citing its 10 for 17 success rate at the time in hit-to-kill intercept tests.

The program got a boost May 30 when the long-awaited — and previously delayed — 18th test to destroy a mock ICBM over the Pacific was successful.

The U.S. missile-defense network in the Pacific includes ground-based missile interceptors at Vandenberg Air Force Base in California and Fort Greely in Alaska; Navy ships armed with a missile-intercept system known as Aegis; and surface-to-air Patriot missile batteries in several locations. Davis said Wednesday that the Pentagon is confident it can shoot down an ICBM now and said the threat posed by North Korea is “nascent.”

“This has been done before,” he said. “It’s something we have mixed results in, but we also have an ability to shoot more than one interceptor.”

Missile defense is only one of several difficult military calculations confronting the Pentagon on North Korea. A conventional U.S. military strike on Pyongyang would have to factor in the hundreds of pieces of artillery and rocket launchers that North Korea has aimed at Seoul, a city of 25.6 million people that is among the most densely packed in the world. The United States also keeps about 28,500 troops in South Korea as a deterrent to a North Korean invasion, and the military frequently says that their motto is that they are “ready to fight tonight” if necessary.

Defense Secretary Jim Mattis, when asked about a potential war with North Korea In May, said during an interview with CBS’s “Face the Nation” that a direct conflict would include “probably the worst kind of fighting in most people’s lifetime,” citing the weapons aimed at Seoul specifically.

“This regime is a threat to the region, to Japan, to South Korea,” said Mattis, who has not issued a public comment since the ICBM launch. “And in the event of war, they would bring danger to China and to Russia as well. But the bottom line is it would be a catastrophic war if this turns into a combat if we’re not able to resolve this situation through diplomatic means.”

Dan Lamothe covers the Pentagon and the U.S. military for The Washington Post. He joined the newspaper in 2014. Lamothe first began covering the military in 2008 and has witnessed Marine sniper operations in southern Afghanistan and Coast Guard patrols in the Arctic Ocean.